1. Field of Invention
The present invention relates to a method for fabricating a semiconductor device, and more particularly, a method for fabricating a semiconductor device by modulating the stresses the semiconductor device is subjected to using the stresses of patterned high stress layers.
2. Related Art
As for the p-type metal oxide semiconductor field effect transistor (PMOS) and the n-type metal oxide semiconductor field effect transistor (NMOS), since the PMOS transistor needs to be applied with compressive stress in the channel area to improve its hole mobility, whereas the NMOS needs to be applied with tensile stress to improve its electron mobility, the forms of stresses to improve the carrier mobility in the channel area are exactly opposite.
Because the magnitude and form of stress of a silicon nitride film, deposited once as a whole, are fixed, in the past, only one of the PMOS and the NMOS transistors on the same wafer could be chosen to apply thereto the stress of a particular form to improve its characteristics. For example, the technique proposed by Taiwan Semiconductor Manufacture Company (TSMC) in the International Electron Devices Meeting (IEDM) in 2003 discloses a method for improving the characteristics of the transistors by means of using stresses in different directions, that is, when the tensile stress in the x-axis direction, i.e. a direction parallel to the channel of the transistor, is increased, the characteristic of the PMOS transistor will be reduced but that of the NMOS transistor can be enhanced; when the tensile stress in the y-axis, i.e. a direction perpendicular to the channel of the transistor, is increased, the characteristics of the NMOS transistor and the PMOS transistor can be enhanced simultaneously. However, when the tensile stress in the z-axis direction, i.e. a direction perpendicular to the surface of the substrate, is increased, the characteristic of the PMOS transistor will be enhanced but that of the NMOS transistor will be reduced. Therefore, to apply stresses in different directions can affect the characteristics of the transistors differently, that is to say, the conventional techniques generally can only improve the characteristic of a single transistor type, but cannot improve the characteristics of the PMOS and the NMOS transistors simultaneously.